The proposed project is directed to develop an experimental thermodynamic characterization of three major aspects of the interactions between membrane associated proteins and phospholipid bilayer systems: 1) The influence of the phospholipid environment on the structure, thermodynamic stability and conformational transitions of membrane proteins; 2) The energetics of protein insertion into membranes, and, 3) The influence of membrane proteins onthe physical state and structural transitions of the phospholipid bilayer matrix. For these studies we have selected three diffeent membrane associated proteins: cytochrome c oxidase, myelin basic protein and diptheria toxin. These proteins are representative of three distinct classes of membrane proteins (integral, peripheral and bivalent) and as such they will allow us to define in precise terms of thermodynamics of their membrane interactions. We will use a novel high sensitivity isothermal reaction microcalorimeter system in conjunction with high sensitivity differential scanning calorimetry. These calorimetric techniques will be complemented by thermal gel analysis, steady state and time resolved fluorescence and phosphorescence spectroscopy, and absorption spectroscopy. These studies will provide fundamental information about the energetics of protein insertion into membranes therefore contributing to our understanding of the self-assembly mechanisms of cellular organelles.